I was working as a partsman in a Ford Dealership between 1979 and 1992, and have a somewhat fuzzy recollection about a TSB (or even recall?) that Ford had issued on (80-82?) 400 truck engines.
They were equipped with a distributor that was not calibrated properly, and causing detonation problems. The fix was a brand new distributor that had been specifically calibrated to address this.
I can't recall the exact tag #, but I think it was E2TE-12127-??. It also had a paper label on the vac advance with "WIN" on it.
The reason that I remember this is that I was having detonation / timing problems with my 351C. The stock single-point, a Mallory dual-point, and a 1974 model Duraspark distributor ALL had timing/detonation issues. Adjusting them "by ear" in order to avoid ping resulted in a "labored" idle and overheating ... even at -30° C frigid Alberta winter temperatures!
I found a guy that had a distributor curving machine, and was getting set to have him overhaul / calibrate it. I was talking to our service manager about my plans, when he mentioned this 400 dist, and offered to let me try one.
It fixed all of my problems!
Here is a post on a Ford Enthusiast's site that also mentions this:
I have a 1979 50 state F-250 4X4 with a 400/C6 combo that I'm the 2nd owner of. It has the original emissions sticker on the hood but the timing spec has been altered to read 4 deg BTDC. Since I know the original owner of this truck and have all the service records my guess is that there was a factory TSB which instructed the technician to change the timing and update the sticker. The reason I think this is because the original part number for the sticker has been similarly altered indicating an updated revision.
On Edit: There were 2 TSBs relating to "Engine Detonation" which came out in 1980 for the 1979 400 motors 80310 and 80310S as well as one in '81 for "Engine - Surging/Stalling/Hard Restart" 811022 and another in '83 for "Distributor - Low Power/Poor fuel economy" 83524. I don't have any other info on them but I would guess they may have had some instructions for adjusting the engine timing.